1. Field of the Invention
This invention relates to a frequency tunable resonator including a varactor (variable capacitance diode) which is widely used in an oscillator of frequencies from VHF to EHF bands.
2. Description of the Related Art
Recently, a resonance circuit combining a dielectric resonator and a varactor has been widely used in oscillators for high frequency wireless apparatuses.
A frequency tunable resonator including a varactor is configured by coupling a dielectric resonator and the varactor via a chip capacitor forming a resonance circuit on a circuit substrate.
FIG. 8 shows a configuration of a typical example of a conventional frequency tunable resonator including a varactor. As shown in FIG. 8, the conventional resonator comprises a dielectric resonator 81, a varactor 82, a printed substrate 83 and chip capacitors 84, 85 and 86. The dielectric resonator 81 is electrically connected to the varactor 82 via the chip capacitor 84. The chip capacitor 85 is a coupling capacitor for coupling an oscillation circuit, which is provided in an external oscillator (not shown), and the frequency tunable resonator including the varactor. The chip capacitor 86 is connected in parallel with the dielectric resonator 81, thereby lowering a resonance frequency. The conventional resonator further comprises a grounded electrode 87, a voltage control terminal 88 and a connection terminal 89 for the oscillation circuit.
Next, the operation of the conventional frequency tunable resonator including the varactor 82 will be explained with reference to FIG. 8. The dielectric resonator 81 is formed by short-circuiting at the end of a coaxial line so as to form quarter-wavelength resonator, and gives an infinite impedance at a resonance frequency. The varactor 82 varies its own capacitance depending upon a D.C. applied voltage, and thus can vary an oscillation frequency of the external oscillator by using this capacitance variation. A variation range of an oscillation frequency, which responds to a variation of D.C. applied voltage, can be varied by changing a capacitance of the chip capacitor 84 which connects the dielectric resonator 81 and the varactor 82. The smaller the capacitance is set, the narrower a variation range of a frequency becomes. On the contrary, the larger the capacitance is set, the wider the variation range of the frequency becomes.
The external oscillator oscillates at a frequency near the resonance frequency of the dielectric resonator 81 on the condition that an impedance of the resonance circuit using capacitances of the varactor 82 and the chip capacitor 84 meets an impedance requirement of the oscillation. Because the oscillation frequency generally shifts from the resonance frequency of the dielectric resonator 81 to a slightly lower frequency, the oscillation frequency is adjusted by cutting the length of the dielectric resonator 81 after mounting the dielectric resonator 81 and the chip capacitor 84 on the printed substrate 83.
However, the above-mentioned conventional frequency tunable resonator including the varactor 82 had some problems that miniaturization of them is difficult and that characteristic adjustment is possible only after mounting both parts on the printed substrate 83, because the dielectric resonator 81 and the varactor 82 are connected via a circuit formed on the printed substrate 83.